This invention relates in general to controllers for electric wheel brakes used on trailers and in particular to an improved controller for such electric wheel brake controllers.
Towed vehicles, such as recreational and utility trailers that are towed by automobiles and small trucks, are commonly provided with electric wheel brakes. The electric wheel brakes generally include a pair of brake shoes which, when actuated, frictionally engage a brake drum. An electromagnet is mounted on one end of a lever to actuate the brake shoes. When an electric current is applied to the electromagnet, the electromagnet is drawn against the rotating brake drum which pivots the lever to actuate the brakes. Typically, the braking force produced by the brake shoes is proportional to the electric current applied to the electromagnet. This electric current can be relatively large. For example, the electric wheel brakes on a two wheeled trailer can draw six amperes of current when actuated and the electric wheel brakes on a four wheeled trailer can draw 12 amperes of current.
Automotive industry standards require that electrically-actuated vehicle wheel brakes be driven against the ground potential of the vehicle power supply. Accordingly, one end of each of the towed vehicle wheel brake electromagnets is electrically connected to the towed vehicle ground and the towed vehicle ground is electrically connected to the towing vehicle ground. The other end of each of the wheel brake electromagnets is electrically connected through an electric wheel brake controller to the towing vehicle power supply.
Various electric brake controllers for towed vehicle electric brakes are known in the art. For example, a variable resistor, such as a rheostat, can be connected between the towing vehicle power supply and the brake electromagnets. Such an actuator is disclosed in U.S. Pat. No. 3,740,691. The towing vehicle operator manually adjusts the variable resistor setting to vary the amount of current supplied to the brake electromagnets and thereby control the amount of braking force developed by the towed vehicle wheel brakes.
It also is known to include an integrating circuit in an electric wheel brake controller. When the towing vehicle brakes are applied, a signal is sent to the integrating circuit. The integrating circuit generates a continually increasing voltage which is applied to the electric wheel brakes. The longer the towing vehicle brakes are applied, the more brake torque is generated by the actuator. A manually adjustable resistor typically controls the rate of integration. One such actuator is disclosed in U.S. Pat. No. 3,738,710.
Also known in the art are more sophisticated electric wheel brake controllers which include electronic circuitry to automatically supply current to the towed vehicle brake electromagnets that is proportional to the towing vehicle deceleration when the towing vehicle brakes are applied. Such electronic wheel brake controllers typically include a sensing unit that automatically generates a brake control signal corresponding to the desired braking effort. For example, the sensing unit can include a pendulum which is displaced from a rest position when the towing vehicle decelerates and an electronic circuit which generates a brake control signal that is proportional to the amount of pendulum displacement. One such unit is disclosed in U.S. Pat. No. 4,721,344. Alternately, the hydraulic pressure in the towing vehicle's braking system or the pressure applied by the vehicle operator's foot to the towing vehicle's brake pedal can be sensed to generate the brake control signal. An example of a controller which senses the towing vehicle brake pressure to generate the brake control signal is disclosed in U.S. Pat. No. 4,398,252.
Known electronic wheel brake controllers also usually include an analog pulse width modulator. The input of the pulse width modulator is electrically connected to the sensing unit and receives the brake control signal therefrom. The pulse width modulator is responsive to the brake control signal to generate an output signal comprising a fixed frequency pulse train. The pulse width modulator varies the duty cycle of the pulse train in direct proportion to the magnitude of the brake control signal. Thus, the duty cycle of the pulse train corresponds to the amount of braking effort desired.
Electronic wheel brake controllers further include an output stage which is electrically connected to the output of the pulse width modulator. The output stage typically has one or more power transistors which are connected between the towing vehicle power supply and the towed vehicle brake electromagnets. The power transistors, which are usually Field Effect Transistors (FET's), function as an electronic switch for supplying electric current to the towed vehicle brakes. The output stage may also include a driver circuit which electrically couples the output of the pulse width modulator to the gates of the FET's.
The output stage is responsive to the pulse width modulator output signal to switch the power transistors between conducting, or “on”, and non-conducting, or “off”, states. As the output transistors are switched between their on and off states in response to the modulator output signal, the brake current is divided into a series of pulses. The power supplied to the towed vehicle brakes and the resulting level of brake application are directly proportional to the duty cycle of the modulator generated output signal.
It is also known to include an manual override control with electronic wheel brake controllers. Such manual override controls typically include a potentiometer that is actuated by a sliding control lever or pushbutton that is moved by the vehicle driver. The potentiometer provides a manual brake control signal to the input of the analog pulse width modulator. The controllers are usually designed to discriminate between the manual brake control signal and the brake control signal supplied by the sensing unit and to respond to the greater signal.